This invention relates to fuel cells and, in particular, to a gas flow control assembly for use in fuel cell systems.
A fuel cell is a device which directly converts chemical energy stored in hydrocarbon fuel into electrical energy by means of an electrochemical reaction. Generally, a fuel cell comprises an anode and a cathode separated by an electrolyte, which serves to conduct electrically charged ions. In order to produce a useful power level, a number of individual fuel cells are stacked in series with an electrically conductive separator plate between each cell.
Fuel cells operate by passing a reactant fuel gas through the anode, while oxidizing gas is passed through the cathode. The electrical output of the fuel cell system depends in part on the rates at which the fuel gas and the oxidizing gas are supplied to, and are carried through, the anode and the cathode, respectively, as well as on the energy content of the fuel. In addition, the efficiency of the fuel cell system depends in part on the fuel utilization, i.e. percentage of fuel utilized by the fuel cell, which is dependent on the fuel's energy content and the fuel flow rate. Conventional fuel cell systems are typically operated at a lower than an optimized fuel utilization, and thus at a lower efficiency, by passing more fuel through the fuel cell than is required for optimized operation in order to avoid starving the fuel cell system of fuel due to variations in the energy content of fuel Therefore, in order to optimize the fuel cell performance and to obtain a desired electrical output and efficiency from the fuel cell system, the energy content of the fuel needs to be monitored and the flow rate at which the fuel is supplied to the cells needs to be precisely controlled.
Conventional fuel cell systems have typically employed mass flow controllers to control the flow rate of the fuel gas to the fuel cell anodes according to the power output of the fuel cell. Conventional systems have also employed an online fuel composition analyzer to determine the composition of the fuel being supplied to the fuel cell system. In particular, mass flow controllers have been used in combination with the online fuel composition analyzer to control the amount of fuel being delivered to the fuel cell based on the fuel composition determination by the composition analyzer and on the power output of the cell.
As can be appreciated, these conventional methods of controlling the flow rate of the fuel to the anodes require the use of complex equipment. In addition, the reliability of the online fuel composition analyzer significantly declines when the fuel being analyzed has a high moisture content and significant variation in composition of the fuel, and also after prolonged operation of the analyzer. Therefore, the analyzer often needs to be re-calibrated to continue to accurately determine the fuel content. Accordingly, a means for controlling the fuel flow to the anode is needed which is accurate and reliable without requiring expensive equipment.
Another assembly and method for controlling the flow rate of fuel to the anode are provided in a commonly-assigned U.S. patent application Ser. No. 11/089,799, which is incorporated herein by reference. The '799 application discloses a control assembly which adjusts the flow of oxidant gas to the cathode based on content variations of anode exhaust gas and controls the flow of fuel to the anode based on the oxidant flow adjustment. Although the assembly disclosed in the '799 application does not require expensive equipment, an improved fuel flow control assembly with faster and more accurate flow adjustment is desired. In addition, means for controlling the fuel flow to the anode that is not affected by ambient temperature is also desired.
It is an object of this invention to provide an improved gas flow control assembly for controlling the flow of fuel to the anode which is reliable, accurate and which provides faster fuel flow adjustment.
It is a further object of the invention to provide an improved gas flow control assembly for controlling the flow of fuel to the anode which is not affected by ambient temperature and which has increased sensitivity to fuel composition variations.